The present invention relates to an ultrasonic probe in which a plurality of transducer elements are arranged with a predetermined pitch so as to form each of channels which transmits an ultrasonic wave to an object to be examined and receives a reflected wave of the transmitted ultrasonic wave. More particularly, the present invention relates to an ultrasonic probe which can enhance a frequency characteristic and reduce a crosstalk between the respective channels, and an ultrasonic diagnostic apparatus using the ultrasonic probe.
A conventional ultrasonic probe, as shown in FIG. 11, includes a plurality of transducer elements 1, 1, . . . which are arranged with a predetermined pitch so as to form each of channels for transmitting an ultrasonic wave and receiving a reflected wave from inside of an object to be examined, electrodes 2a, 2b provided on a front surface and a rear surface of each of the transducer elements 1, 1, . . . so as to apply a voltage thereto, and an acoustic matching layer 3 for taking a matching of an acoustic impedance between the transducer elements 1, 1, . . . and the object to be examined. In FIG. 11, a reference numeral 4 denotes a backing material for preventing an ultrasonic wave transmitted from the rear surface of the transducer element 1 from getting back again to the rear surface thereof. A reference numeral 5 denotes an acoustic lens provided over the above-described acoustic matching layer 3 so as to focus an ultrasonic beam transmitted from the front surface of the transducer element 1.
Here, concerning the acoustic matching layer 3, an acoustic impedance thereof is set to be an intermediate value between an acoustic impedance of the transducer element 1 and that of the object to be examined. Moreover, the acoustic matching layer 3 is formed so that a thickness thereof becomes equal to 1/4th of a wavelength of the ultrasonic wave. If the acoustic matching layer 3 as described above is continuously arranged in a transverse direction along the direction in which the transducer elements 1, 1, . . . are arranged, a crosstalk between the channels (an radio interference between signals) is increased due to the ultrasonic wave propagating through the acoustic matching layer 3. Accordingly, incisions are made into the acoustic matching layer 3 in correspondence with gaps 6, 6, . . . between the channels of the plurality of transducer elements 1, 1, . . . arranged with a predetermined pitch, thereby forming gaps 7, 7, . . . between the channels to reduce the crosstalk.
However, in such a conventional ultrasonic probe, since the above-mentioned acoustic matching layer 3 is formed using a uniform material having an acoustic impedance of an intermediate value between the acoustic impedance of the transducer element 1 and that of the object to be examined, the acoustic matching layer 3 does not perform a pure piston movement in only a direction in which an ultrasonic wave is transmitted from the transducer element 1, but oscillates also in a direction perpendicular to the transmitted direction of the ultrasonic wave. The existence of such an oscillation mode in the direction perpendicular to the transmitted direction of the ultrasonic wave in each acoustic matching layer 3 is harmful to an original purpose of the ultrasonic wave transmission/reception. The reason is that, if an ultrasonic wave is transmitted at a frequency close to the oscillation frequency in the transverse direction, a part of the energy thereof is used for the oscillation of the acoustic matching layer in the transverse direction, so that an original response in the longitudinal direction is deteriorated.
For example, as is seen in a simulation on Round Trip Impulse Response shown in FIG. 12, there occurred an irregular tail-trailing c in the echo voltage waveform with a lapse of time, so that a pulse characteristic of a reception signal becomes worse. Also, as is seen in a simulation on Round Trip Frequency Response shown in FIG. 13, there occurred ripples d.sub.1, d.sub.2, and d.sub.3 at certain frequencies, so that a broad frequency characteristic is not obtained. On account of this, in the conventional ultrasonic probe, it is not able to optimize the oscillation mode in the direction perpendicular to the transmitted direction of the ultrasonic wave in the acoustic matching layer 3. This eventually brought about no broad frequency characteristic or no excellent pulse response characteristic, thus, in some cases, resulting in a deterioration in image quality of an ultrasonic wave image.